This invention relates to a conductive sheet and an electrostatic recording medium formed therefrom.
Known conductive sheets include those prepared from an organic polymer incorporated with a conductive powder such as carbon powder or metal powder, or those formed by applying an inorganic conductive layer of aluminum, silver, gold, tin oxide-indium oxide on the surface of an organic polymer by vacuum deposition or sputtering. Further, electrostatic recording media prepared by forming a dielectric layer on a conductive sheet are also known.
Generally, the surface resistivity required for conductive layers of electrostatic recording media falls within the range of 10.sup.4 to 10.sup.9 ohm per square.
To obtain stable image properties, it is believed desirable above all, though it depends on the electrostatic recording system used, that the uniformity, i.e., the dispersion about the median value, of the surface resistivities of a conductive layer be within about .+-.20%, and that the change with time, i.e., the ratio of the value after the lapse of time to the initial value be within 3 to 5.
However, in these conventional conductive sheets, their surface resistivities depend largely on the amount of a conductive powder added or the amount of an inorganic conductive layer formed, and especially when the surface resistivity of a conductive sheet is within the semiconductor region of 10.sup.4 to 10.sup.9 ohm per square, its value changes greatly with even a slight change in the amount of the conductive powder or the inorganic conductive layer. Therefore, there has been a problem that it is difficult to obtain a resistivity which is uniform over a wide area.
Further, a conductive sheet having an inorganic conductive layer on the surface has a problem that when it is exposed to air or water vapor for a long time, or placed in a high-temperature and high-humidity atmosphere, its electric resistance increases greatly.
Therefore, the conventional conductive sheets can not be used in the field where that electrical conductivity is necessary which is uniform and stable and within the semiconductor region of 10.sup.4 to 10.sup.9 ohm per square. Therefore, electrostatic recording media prepared by using the conventional conductive sheets have serious obstruction in putting them into practical use because they have problems that because of a great change in the resistance value of a conductive layer, no image which is uniform over a wide area can be obtained, and that when they are exposed to air or water vapor for a long time, or placed in a high-temperature and high-humidity atmosphere, their surface resistance values increase and therefore no stable image properties can be obtained.